The present invention relates to structures for use in the filtration of exhaust streams, particularly for the capturing of particulate matter. Specifically the invention is a multicellular structure, particularly a honeycomb with differing wall thickness or differing heat capacity along the flow axis, especially suited for diesel particulate filters.
Standard commercially available filters are made of cordierite (2MgO-2Al2O3-5SiO2). Cordierite has a low coefficient of thermal expansion (xcx9c5-8xc3x9710xe2x88x927/xc2x0 C.), good thermal shock and a moderately high melting point (xcx9c1460xc2x0 C.). These properties coupled with the low cost of the cordierite-forming raw materials (e.g., clay, talc, alumina, and silica) are what have made cordierite a desirable candidate for diesel particulate filters. However, in some applications such as diesel particulate filters (DPFs), which require regeneration of the trapped carbon soot, the combustion process may produce temperature spikes which may exceed the melting point of cordierite and can thermally shock and crack, or even melt the filter. Therefore a significant problem associated with conventional cordierite DPFs is susceptibility to damage during the required filter regeneration cycling.
A great deal of effort has been expended to try to find a material that has a higher use temperature than cordierite with equivalent thermal shock resistance. No material with the same ease of production, cost and properties, especially thermal expansion coefficient has yet to be found.
Recently silicon carbide has been proposed for use in DPFs. SiC has a significantly higher melting point (2750xc2x0 C.) than cordierite. However disadvantages of SiC include excessive die wear and difficulties in sintering, all translating into high costs associated manufacture of DPFs, which have led to limited commercial use. Silicon carbide DPFs are made from approximately one inch square DPF type honeycomb segments cemented together with an inorganic cement containing inorganic fibers.
Other high temperature materials, such as NZP, have been proposed but excessive die wear, difficulties in sintering, phosphorous loss, etc. have led to very limited commercial use.
In the art it is known that increased thermal mass in the body of the part is desirable for DPFs made of materials that can melt at intermediate temperatures such as cordierite, while still providing adequate low back pressure and reasonable soot loading capacity. However, if the part is too massive, then regeneration is difficult to initiate, costing more energy input, usually fuel, to achieve regeneration light-off.
With most filter designs, the light off occurs on the front face of the DPF and the maximum temperatures occur on the back face of the part. Therefore, it would be advantageous, through filter design to minimize the energy cost of initiating the regeneration, while not promoting excessive temperature during regeneration on the down stream end of the filter.
In one embodiment the invention is a multicellular structure such as a honeycomb structure with differing wall thickness along the flow axis, particularly suited for diesel particulate filters (DPFs), however, it is also applicable to flow-through honeycombs. The honeycomb can be a single monolith or can be of a segmented DPF type. A preferred embodiment is a honeycomb, particularly for diesel engines or automobile engines, where the body has thinner walls on one end of the honeycomb and thicker walls on the other. For a DPF, the thinner walls on the front surface allows the DPF to initiate regeneration with less energy cost. For automotive honeycombs, a front with thinner walls (a lower mass front) allows earlier light-off. The honeycomb can be made of any suitable ceramic such as cordierite or silicon carbide.
The honeycomb structure has a heat capacity ranging from above 8.5xc3x9710xe2x88x923cal/cubic cm-K to below 0.25 cal/cubic cm-K along an axis from the inlet end to the outlet end.
The invention is also using solid freeform manufacturing, particularly binder ink jet printing to produce this structure.